The present invention relates to a method of coating substrates which comprises
a. applying a flowable coating composition to the target substrate and then
b. curing the resulting wet coating by exposure to high energy radiation.
The production of coatings by applying a flowable coating composition to the target substrate (i.e., the substrate that is to be coated) and then curing the resulting wet coating by exposure to high energy radiation is known from the prior art: for example, from P.K.T. Oldring et al. in Chemistry and Technology of UV and EB Formulation for Coatings, Inks and Paints, SITA Technology, London (1991).
The coating compositions used in such techniques generally comprise organic polymers or prepolymers containing two or more ethylenically unsaturated double bonds or epoxy groups per molecule. On exposure to high energy radiation, e.g., UV radiation, of the substrate provided with the coating composition, these reactive groups undergo photopolymerization, in the course of which a polymeric network is formed in the as yet uncured coating. In this way, a solid coating is obtained. Radiation-curable coatings is a term used in this context. Accordingly, the polymers or oligomers that are present in these coating compositions, containing reactive groups, are also referred to as radiation-curable binders. In order to achieve sufficient strength in the coating, such polymers are required to include a certain amount of reactive groups, generally not less than 2 mol/kg of polymer (or oligomer).
Radiation-curable coatings are frequently used to coat wood or paper, for woodblock flooring or in the furniture industry, for example. There have also been reports on the use of radiation-curable coating compositions in the form of UV-curable inks, as for example in EP-A 658 607, JP-A 48922/96 and WO 99/01517. The inks described therein comprise as binders for the pigments a radiation-curable binder system based on ethylenically unsaturated polymers which on exposure to UV radiation undergo photopolymerization and so fix the pigments on the printed surface.
One of the advantages of radiation-curable coatings is their high mechanical stability. In addition, radiation-curable coating compositions can be processed very much more rapidly than their conventional counterparts, since the coating cures by a rapid photopolymerization rather than by a slow drying process or thermal crosslinking reactions. However, it is in the reactive nature of these coating compositions that they must be stored and processed in the absence of light and are also of only limited stability with respect to oxygen. Moreover, the adhesion of such coating compositions to smooth, nonswellable surfaces is often inadequate.
Conventional inks comprising polyvinylbutyral as their binder are known from DE-A 33 15 741. Japanese Laid-Open Specification H8-239614, moreover, proposes polyacetals as cobinders in thermally curable inks. Such inks exhibit poor adhesion to printed substrates, especially to plastics or surface coatings.
It is an object of the present invention, in view of the fundamental advantages of radiation curing for the production of coatings, to provide a further method of producing coatings using high energy radiation. It is also intended in particular that the method should be suitable for printing substrates and should overcome the disadvantages of the prior art.
We have found that this object is achieved by means of a method wherein the binder used comprises a polymer containing less than 1 mol/kg of reactive groups, selected from ethylenically unsaturated double bonds and epoxy groups, and at the same time at least 2 mol/kg of functional groups of the formula I defined below.
Accordingly, the present invention provides a method of coating, especially printing, substrates and with particular preference of producing thin films having dry film thicknesses of on average not more than 10 xcexcm, including the following steps:
a. applying a flowable coating composition to the target substrate and
b. curing the resulting wet coating by exposure to high energy radiation,
wherein said coating composition comprises as binder a polymer A containing less than 1 mol/(kg of polymer A) of reactive groups and at least 2 mol/(kg of polymer A), preferably 3 mol/kg, in particular 4 mol/kg, with particular preference 5 mol/kg, of functional groups of the formula I 
where Y is Nxe2x80x94H or oxygen, embodied by the substructures of the formula IA, IB or IC 
where
R and Rxe2x80x2 independently of one another are hydrogen or C1-C4-alkyl
Rxe2x80x3 is C1-C10-alkyl,
Y is as defined above, and
n is 0 or 1,
the structural units of the formula IA, IB or IC being constituents of the polymer chain.
The term xe2x80x9ccoatingxe2x80x9d as used here embraces printing, enscription, or coating in any other way. Substructures and functional groups for the purposes of the invention are defined groups of atoms in the polymer A.
The structural elements of the formulae IA, IB and IC comprise ether groups, amino groups, acetals, ketals, aminals and hemiaminals and also the alcohol or amine moieties in esters or amines that carry a hydrogen atom in each case on at least one carbon atom adjacent to the heteroatom.
Among the functional groups of the formulae I, preference is given to those structural elements in which Y is oxygen. R in formula IA is preferably hydrogen. In formula IC, at least one of the two radicals R and Rxe2x80x2 is hydrogen. Preferred polymers A are those containing structural units of the formula IC, and in turn those in which Y is oxygen.
Polymers A generally have a weight-average molecular weight of 1000, preferably at least 2000, in particular at least 5000, e.g., from 5000 to 5,000,000.